The present invention relates to signal processing of modulated signals, including audio and higher frequency signals transmitted by various modes, such as radio, television and line systems and especially for processing narrow band voice signals to increase intelligibility. The invention is particularly concerned with digital signal processing methods for permitting transfer of signals, such as voice and data, in a manner which provides maximum efficient transfer of signal energy and intelligence.
When transmitting intelligence-carrying signals, especially modulated signals (e.g., audio modulated radio frequency signals), there has always been a problem of loss or attenuation of intelligence attempted to be transferred within a given frequency band where the frequency band over which intelligence must be transferred is narrower than the range of frequencies which normally convey such intelligence. Thus, when the bandwidth or deviation of a radio frequency signal is narrow, the full spectrum of speech and other audio signals has been heretofore limited. It has, therefore, been general practice merely to transfer only a portion of those frequencies of the audio spectrum with consequent loss of intelligibility, voice character, dynamics, clarity and fidelity in general. In particular, for both line and radio frequency transmission and reception of modulated signals, a major problem has always been to obtain a high level of dynamic amplitude while retaining a usable portion of the full spectrum of speech frequencies, while keeping the bandwidth of the transmitted radio signals as narrow as possible.
In the transmission of voice signals, particularly within a narrow pass band such as 300 Hz-3 kHz used for narrow band radio transmission (e.g., AM, SSB and FM) there is often a problem of loss of dynamic amplitude and harmonic relationship upon receiving the transmitted signals. The losses result from the suppression of higher frequency order components of the signal, and particularly out-of-band harmonics which define voice character and, thus, intelligibility. Conventional compression of voice signals for radio transmission has long been utilized, but results in loss of voice dynamics with consequent loss of normal voice character and intelligence.
In Bloy U.S. Pat. Nos. 4,368,435 and 4,457,014, and in U.S. application Ser. No. 06/364,931, U.S. Pat. No. 4,741,039 of the same inventor, there are disclosed analog systems for processing of such voice signals in such a way that dynamic amplitude and harmonics behind the pass band are preserved and reconstituted during transmission to permit voice character and intelligibility to be retained in the transmitted signals, with the result that the signals may be transferred with reduced bandwidth.
Similarly, transmission of other types of signals results in their degradation as transmitted through narrow pass bands or under conditions wherein high frequency constituents may be lost during transmission, modulation, demodulation or otherwise because of the bandwidth limitations of the transmission system. For example, data may be represented by Fourier transformation and expansion into a series of sinusoidal signals having harmonics beyond the pass band capabilities. Demonstrably, narrow band transmission limits the upper frequency and rate of transmission of such signals. Therefore, it is desired also to be able to retain the intelligence which otherwise is lost during narrow band line and radio transmission.
For both analog and digital signal transmission, there are many modes of radio and line transmission where such matters of high frequency and harmonic suppression, resulting in loss of intelligibility or otherwise resulting in degradation of signal character, are of concern.
Copending U.S. application Ser. No. 856,832, U.S. Pat. No. 4,695,825 of Graham P. Bloy and of John L. Miramonti discloses an analog-digital conversion system for shortening the interval with at least one of successive timing states associated with repetitive cycles of a digital processing system, such as that herein disclosed and claimed by said analog-digital conversion system may be used for implementing the present digital signal processing methods.